Excerpts from NASA Procedural Requirements, NPR 8715.3B (effective April 04, 2007):
c. Not use hand-held communication devices while the vehicle is motion except for emergency, security, and fire vehicles during official operations.
Note: This includes cell phones, UHF radios, or other hand-held wireless communication devices.
For complete regulation, see http://nodis3.gsfc.nasa.gov/displayDir.cfm?Internal_ID=N_PR_8715_003B_&page_name=Chapter3
Note: Above regulation is applicable to federal property, NASA Ames Research Center, not outside the Ames boundary.
The new California Vehicle Code cellphone law, CVC 23123 effective July 1, 2008, prohibits cellphone use but not two-way radios including amateur radio units. See http://www.dmv.ca.gov/cellularphonelaws/. It is legal to operate your amateur radio while driving but with common sense of course. You still can be cited for unsafe driving.